1. Field of the Invention
The present invention relates generally to a packet receiving method and apparatus in a mobile communication system. In particular, the present invention relates to a method and apparatus for receiving downlink rate-matched packet data from a Node B and de-rate matching the received packet data at a User Equipment (UE) in a High Speed Downlink Packet Access (HSDPA) system.
2. Description of the Related Art
Mobile communication systems typically adopt multiple access techniques. Multiple access systems comprise Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA). In FDMA, a total frequency band is divided into a plurality of channels, each channel being allocated to one user. In TDMA, a plurality of users share the same frequency channel at different times. CDMA allows a plurality of users to simultaneously access the sane frequency band. The users for the various systems are separated because they use different codes for communications. With today's rapid development of communication technology, these mobile communication systems have reached the point where they can provide packet data service or packet service which covers high-speed, high-quality digital data transmission such as moving pictures, and multimedia service, in addition to the standard voice service.
Mobile communication systems typically use CDMA for the provisioning of the packet data service. Major high-speed packet data transmission systems based on CDMA include High Speed Downlink Packet Access (HSDPA) proposed by 3rd Generation Partnership Project (3GPP), and Evolution-Data Only (EV-DO) and Evolution-Data and Voice (EV-DV) which are synchronous systems proposed by 3rd Generation Partnership Project 2 (3GPP2). HSDPA is a standard that was proposed on May, 2002 to enable high-speed downlink packet transmission in Universal Mobile Telecommunication Systems (UMTSs), and specifies the configuration of transport and control channels associated with high-speed packet transmission to UEs and their operation.
Meanwhile, HSDPA regulates that a UE, after receiving a packet from a Node B, notifies the Node B as to whether the reception is successful or not. In the absence of errors in the received packet, the UE requests the retransmission of the packet by a link control protocol Hybrid Automatic Repeat Request (HARQ). Because receiving packets free of noise or distortion over a radio network at a UE is impossible to physically achieve, many packet retransmission techniques were introduced to HARQ to overcome this problem.
The packet retransmission techniques include Chase Combining (CC), Full Incremental Redundancy (FIR), and Partial Incremental Redundancy (PAIR). CC retransmits the same packet as that of the first attempt. A UE receiver combines the retransmitted packet with the buffered original packet. The resulting increased reliability of coded bits at the input of a channel decoder improves the whole system performance gain. The FIR scheme retransmits a completely different packet from the first one. The retransmitted packet comprises only parity bits produced in a channel encoder, thereby yielding better packet transmission performance by reducing a coding rate. The PIR scheme retransmits a partially different packet from the first one. The retransmitted packet is a combination of systematic bits and parity bits.
HARQ packet retransmission involves rate matching in a Node B transmitter and de-rate matching in a UE receiver. In most cases, the number of coded bits in the transmitter is different from that of the bits of a transport unit (TU) actually transmitted over a radio network. The rate matching refers to adaptation of the number of coded bits to that of bits suitable for transmission over a radio network through repetition or puncturing, prior to transmission. The de-rate matching is the reverse of rate matching before decoding repeated or punctured data received from the transmitter.
With reference to FIG. 1 to FIG. 4, the typical HARQ packet transmission/retransmission will be detailed.
FIG. 1 is a block diagram of a rate matching apparatus in a conventional HSDPA system. The rate matching apparatus is provided in a transmitter of, for example, a Node B. The rate matching occurs in each HARQ processor.
Referring to FIG. 1, a rate matcher 100 separates a signal RIN having coded bits received under the control of a controller 200 into systematic bits SB, first parity bits P1, and second parity bits P2, primarily matches the total number of the bits of SB, P1 and P2 to a predetermined bit number, and secondarily matches the first-rate matched bit number to the number of bits transportable on a physical channel over a radio network. After the second rate matching, the coded bits are subject to interleaving and modulation prior to transmission.
For the above operation, a bit separator 110 separates RIN into SB, P1 and P2. A first matcher 120 primarily rate-matches SB, P1 and P2 using a virtual buffer 130 of which the size is set to a predetermined value in each HARQ processor. In the first rate matching, if the total data size of SB, P1 and P2 exceeds the size of the virtual buffer 130, P1 and P2 are punctured to match the buffer size by using a predetermined rule.
A second matcher 140 readjusts the number of the bits received from the first matcher 120 and the virtual buffer 130 to that of bits transportable on the physical channel over the radio network. In the second rate matching, if the number of the first rate-matched bits is less than the bit number of the TU of the physical channel, predetermined bits are repeated in a predetermined rule. If the number of the first rate-matched bits is larger than the bit number of the TU, the predetermined bits are punctured.
At a packet retransmission, the pattern of the second rate matching is changed, that is, a so-called hybrid retransmission is performed. Hybrid retransmission refers to a rate-matching pattern change such as changing rate-matched bits, puncturing of parity bits mainly with a higher priority given to system bits, or puncturing of system bits mainly with a higher priority given to parity bits.
A bit collector 150 rearranges the second rate-matched packet bits to an interleaver size, for interleaving. The interleaved packet is modulated and transmitted over the radio network. In this way, the configuration illustrated in FIG. 1 adapts the data size of a transmission/retransmission packet to that suitable for the physical channel through first and second rate matchings in the transmitter, prior to transmission to the receiver.
Regarding packet retransmission involving the first and second rate matchings, several retransmission techniques can be employed for initial transmission and retransmission such as transmission of system bits only, transmission of the system bits, and first and second parity bits, and transmission of the system bits and partial first and second parity bits.
FIG. 2 is a block diagram of a de-rate matching apparatus in the conventional HSDPA system. The de-rate matching apparatus is provided in a receiver of, for example, a UE. The de-rate matching is the reverse of the rate matching. The de-rate matching apparatus comprises a de-rate matcher 300 for combining an initial transmitted packet with a retransmitted packet and writing the combined data in a combining buffer 340, and a controller 400 for providing overall control to the operation of the de-rate matcher 300.
Referring to FIG. 2, the controller 400 decodes a High Speed Shared Control Channel (HS-SCCH) signal received along with packet data over the radio network, interprets predetermined HARQ parameters, and determines whether the packet data is an initial transmitted packet or a retransmitted packet. It also determines whether to combine data according to the determination. In the case of an initial transmitted packet, a collection buffer 310 of the de-rate matcher 200 temporarily buffers an input signal DIN from a demodulator (not shown).
In relation to de-rate matching, a zero generator 320 outputs zeroes under the control of the controller 400 so that the zeroes can be inserted in punctured positions. A multiplexer (MUC) 330, of which the input port is connected to the collection buffer 310 and the zero generator 320, selectively outputs to the combining buffer 340 the data received from the collection buffer 310 or the zeroes received from the zero generator 320 according to a predetermined select command.
The combining buffer 340 stores the de-rate matched data at predetermined addresses indicated by an address generator 360. The controller 400 determines the punctured positions in DN from the HARQ parameters and controls the address generator 360 to generate the address at which the data from the collection buffer 310 is stored and an address at which the zeroes from the zero generator 320 are stored.
The address generator 360 generates addresses, starting from the de-rate matching. The number of the addresses is identical to the bit number of the data including the zeroes buffered in the combining buffer 340. The conventional de-rate matching performed in the configuration of FIG. 2 will be described below with reference to FIGS. 10A and 10B. FIG. 10A illustrates an example of data buffered in the collection buffer 310 before de-rate matching and FIG. 10B illustrates de-rate matched data buffered in the combining buffer 340. It is assumed herein that data is punctured at positions #5, #10, #15, #20, and so on during the second rate matching in the configuration of FIG. 1.
In this case, data #1, #2, #3 and #4 in the collection buffer 310 are stored as they are in the de-rate matching, as illustrated in FIG. 10B. Since #5 is set as a punctured position in the combining buffer 340, zero generated from the zero generator 320 is stored at the position #5. Therefore, data #5 in FIG. 10A is stored as data #6 in FIG. 10B and the following data is de-rate matched and stored in the same manner.
Meanwhile, when a retransmitted packet is received at the de-rate matching apparatus illustrated in FIG. 2 according to the HARQ protocol operation, the retransmitted packet is provided to a combiner 341 through the collection buffer 310 and the MUX 330 under the control of the controller 400. The initial transmitted packet is retrieved from the combining buffer 340 and provided to the combiner 341. The initial transmitted packet is combined with the retransmitted packet in the combiner 341 and then stored in the combining buffer 340. As stated above, zeroes are inserted at punctured positions in the storing operation.
The above-described de-rate matching apparatus recovers a packet transmitted from the rate matching apparatus of the transmitter over the radio network to the original data before rate matching. Notably, this conventional de-rate matching apparatus is configured to support the FIR scheme in that a packet received on a physical channel and zeroes in place of punctured data are sequentially stored in the combining buffer 340 in de-rate matching. Since addresses are generated at which the data are stored and at the same time, zeroes are inserted at punctured addresses in the course of buffering the data of a received packet, the de-rate matching is time consuming.
With reference to FIGS. 3 and 4, the time required for the typical rate matching and de-rate matching will be addressed.
FIG. 3 is a diagram illustrating the transmission timings of channel information transmitted/received between a Node B and a UE in packet transmission in the typical HSDPA system. In the illustrated case, an HSDPA packet is delivered on a 3-slot basis.
Upon generation of a downlink packet, the Node B transmits control information including HARQ parameters to the UE on a HS-SCCH. Two slots later, the Node B transmits a packet to the UE on a High Speed Downlink Shared Channel (HS-DSCH). The UE subjects the HS-DSCH packet to demodulation, de-rate matching and decoding. Then, it transmits to the Node B an ACK/NACK signal indicating whether the downlink packet has reception errors or not, and a Channel Quality Indicator (CQI) on a High Speed Dedicated Physical Control Channel (HS-DPCCH). The Node B determines whether to retransmit the packet based on the ACK/NACK information.
In FIG. 3, T1 denotes a time point at which decoding of the HS-SCCH information is completed and T2 denotes a time point at which the HS-DSCH is completely received. Decoding of the received packet is completed at time T3. The HSDPA standard recommends that the time between T2 and T3 is 7.5 or less slots (5 ms or less). This implies that the UE receiver, after receiving a downlink packet, must complete de-rate matching and packet decoding within the time of 7.5 slots.
FIG. 4 illustrates the time required for de-rate matching in the conventional HSDPA system. In the figure, the processing time in each input buffer (not shown) and each interleaving buffer (not shown), the collection buffer 310 and the combining buffer 340 for de-rate matching, and a decoding buffer (not shown) is sequentially illustrated between T2 and T3. This corresponds to the decoding time of UE category 10 in 3GPP Release 15, for example.
Referring to FIG. 4, the de-rate matching between the collection buffer 310 and the combining buffer 340 occupies 40% or more of 7.5 slots (5 ms), 2.736 ms. The de-rate matching apparatus illustrated in FIG. 2 does not satisfy the time requirement of 7.5 or less slots. Moreover, UEs of higher categories including UE category 7 do not satisfy the time requirement, either. While the problem can be solved by using an additional buffer that reduces the de-rate matching time through distributed processing of a received packet, or by increasing the frequency of a packet transmission clock, the former method makes UE configuration complex and increases product cost, and the latter method increases UE power consumption.
Therefore, there is a need for reducing the time required for de-rate matching of a received packet in an HSDPA system.